Movie projectors have used pin registered film gates for many years. Cinema projectors typically use a pair of registration pins such as those shown in FIG. 1 (from the paper "Pin Registration" by A. C. Robertson, Journal of the SMPTE, Vol. 72, February 1963, pp. 75-81). The dimensions of the "big pin" 10 are closely matched to the dimensions of the perforation. Big pin 10 is intended to be "full-fitting" with respect to the perforation, and thus prevent motion of the film in two degrees of freedom. The "little pin" 12 is full-fitting only in the film's "in-track" direction, thus constraining the film's third degree of freedom. Unfortunately, it is impossible to have a perfect match between the pin dimensions and the perforation dimensions, so the pins either fit too tight (producing wear of the pins and damage to the perforations) or too loose (resulting in "play" and imprecise film positioning). This latter problem manifests itself as image motion on the movie screen. This small amount of motion, while detectable, has long been considered acceptable. Recently, however, owing to the increased use of special effects, where several images are superimposed to form a composite image, there is now a need to be able to very precisely and repeatably reposition the film in the gate so that each of these images will be properly aligned without variation.
In U.S. Ser. No. 903,837, an improved pin registered film gate was disclosed. In this disclosure, neither of the registration pins was full-fitting with respect to its perforations. Instead, the two pins (considerably undersize relative to their perforations), were inserted into the perforations and a nesting force was then subsequently applied to cause the film to move so that the edges of the perforations would become nested against the pins. The position and angle of the nesting force was shown to be equal to the vector sum of the three individual forces occurring at the three points of contact between the two registration pins and the perforations. This pin registration technique overcame the problem of imprecision of conventional pin registered film gates which tried to achieve "perfect" match between the dimensions of the pins and those of the perforations.
U.S. Ser. No. 903,689 has disclosed a similar means of accomplishing pin registration except that the nesting force is applied by a spring-loaded pin which is inserted into a third perforation simultaneously with the insertion of the registration pins. Also, the design and operation of a film clamp is described. This film clamp operates by pressing the film against the cylindrical surface of the gate, starting first at the area of the film adjacent to the registration pins, then "ironing out" the film by progressively wrapping the clamp around the cylindrical gate. This progressive wrapping action, while suitable for slow speed operation, would be far too time consuming for a gate which is to operate at several frames per second.
The present invention is intended to achieve the same high degree of precision achieved by the pin registration means of U.S. Ser. No. 903,837 and U.S. Ser. No. 903,689, but without making the same sacrifice in speed of operation.